Group III-V nitride compound semiconductors such as gallium nitride (GaN) have wide bandgap. Exploiting this feature, they are applied to light-emitting diodes (LED) emitting ultraviolet to blue/green light with high brightness, and laser diodes (LD) emitting blue-violet to blue light.
These light-emitting devices have the following structure. On a sapphire substrate, for instance, an n-type semiconductor layer, a light-emitting layer including a quantum well layer and a barrier layer, and a p-type semiconductor layer are stacked in this order.
In such semiconductor light-emitting devices, there is demand for simultaneously achieving low driving voltage and high light emission efficiency.
By thinning the barrier layer, the driving voltage tends to decrease. However, thinning the barrier layer results in degraded crystallinity, which decreases the light emission efficiency. On the other hand, the quantum well layer is made of e.g. InGaN. Here, nonuniform composition ratio of In and difference in lattice constant cause lattice strain in the quantum well layer. As a result, defects are generated in the crystal. Furthermore, the piezoelectric field induced by the strain modulates the band structure of the active layer and decreases the light emission efficiency.
With a view to increasing the light emission output power, decreasing the forward voltage, and improving the electrostatic breakdown voltage, a nitride semiconductor device is described in JP-3424629. This nitride semiconductor device includes an active layer between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. The n-type nitride semiconductor layer includes an n-type contact layer and an n-type multi-film layer with a superlattice structure. Furthermore, an undoped GaN layer having a film thickness of 100 angstroms or more is interposed between the n-type contact layer and the n-type multi-film layer. However, despite such conventional techniques, there is room for improvement to achieve low driving voltage and high light emission efficiency.